Nowadays, from the viewpoint of an increase in the fuel efficiency of automobiles and the collision safety of automobiles, there is a growing demand for weight reduction and strengthening of automobile bodies by increasing the strength of a material for automobile bodies in order to decrease the thickness of the material. Therefore, the application of high-strength steel sheets to automobiles is promoted.
Generally, an automotive steel sheet is used in the painted state, and a chemical conversion treatment called phosphating is performed as a pretreatment for such painting. The chemical conversion treatment of a steel sheet is one of the important treatments for achieving corrosion resistance of the steel sheet after painting has been performed.
It is effective to add Si and Mn in order to increase the strength and the ductility of a steel sheet. However, when continuous annealing is performed, Si and Mn oxidize and form surface oxides selectively containing Si and Mn (such as SiO2 and MnO, referred to as “selective surface oxides” hereinafter) in the outermost surface layer of the steel sheet even in a reducing atmosphere of N2+H2 gas in which oxidation of Fe does not occur (that is, oxidized Fe is reduced). Since such selective surface oxides inhibit the generation reaction of a chemical conversion coating when a chemical conversion treatment is performed, a micro region in which a chemical conversion coating is not formed (also referred to as a “lack of hiding” hereinafter) is formed, which results in a decrease in phosphatability.
Patent Literature 1 discloses an example of conventional techniques for increasing the phosphatability of a steel sheet containing Si and Mn in which an iron coating layer having a coating weight of 20 to 1500 mg/m2 is formed on a steel sheet by using an electroplating method. However, in the case of this method, since additional electroplating equipment is needed, there are problems of an increase in the number of processes and an increase in cost.
In addition, in Patent Literature 2, phosphatability is increased by specifying the ratio of Mn to Si (Mn/Si). In Patent Literature 3, phosphatability is increased by adding Ni. However, since such effects depend on the contents of Si and Mn in a steel sheet, it is considered that further improvement is necessary in the case of a steel sheet having high Si and Mn contents.
Patent Literature 4 discloses a method in which, by controlling the dew point to be −25° C. to 0° C. when annealing is performed, an internal oxide layer including oxides containing Si is formed within 1 μm from the surface of a bare steel sheet in the depth direction so that Si-containing oxides constitute 80% or less of a length of 10 μm on the surface of a steel sheet. However, since the method according to Patent Literature 4 is based on the assumption that the zone in which the dew point is controlled is the whole furnace interior, it is difficult to control the dew point, and, as a result, it is difficult to realize a stable operation. In addition, in the case where annealing is performed while the dew point is unstably controlled, since there is a variation in the distribution of internal oxides formed in the steel sheet, there is concern that an irregularity in the result of a chemical conversion treatment or a lack of hiding may occur in whole or part in the longitudinal direction or width direction of the steel sheet. Moreover, even in the case where there is an increase in phosphatability, since Si-containing oxides exist immediately under a chemical conversion coating, there is a problem of poor corrosion resistance after electrodeposition coating has been performed.
Patent Literature 5 describes a method in which a steel sheet is heated to a temperature of 350° C. to 650° C. in an oxidizing atmosphere in order to form an oxide film on the surface of the steel sheet, then heated to the recrystallization temperature in a reducing atmosphere, and then cooled. However, in the case of this method, since the thickness of the oxide film formed on the surface of the steel sheet varies depending on an oxidizing method, there is a case where oxidizing does not sufficiently progress or where the thickness of oxide film formed is so thick that the oxide film is retained or flaking of the oxide film occurs when annealing is subsequently performed in a reducing atmosphere, which may result in a decrease in surface quality. In addition, in the EXAMPLES of Patent Literature 5, a technique in which oxidation is performed in atmospheric air is described. However, in the case of oxidation in atmospheric air, since a thick oxide layer is formed, there is a problem, for example, in that it is difficult to subsequently perform reduction or in that a reducing atmosphere having a high hydrogen concentration is needed.
Patent Literature 6 describes a method in which a cold-rolled steel sheet containing, by mass %, 0.1% or more of Si and/or 1.0% or more of Mn is heated to a temperature of 400° C. or higher in an iron-oxidizing atmosphere in order to form an oxide film on the surface of the steel sheet, and the oxide film on the surface of the steel sheet is subsequently reduced in an iron-reducing atmosphere. Specifically, by oxidizing Fe on the surface of a steel sheet at a temperature of 400° C. or higher by using direct fire burners in an atmosphere having an air ratio of 0.93 or more and 1.10 or less, and by then annealing the steel sheet in an atmosphere of N2+H2 gas for reducing Fe oxide, the oxidation of Si on the outermost surface, which decreases phosphatability, is inhibited so that an Fe oxide layer is formed on the outermost surface. Although the heating temperature of the direct fire burners is not specifically described in Patent Literature 6, it is considered that, in the case where the Si content is high (about 0.6% or more), since Si is more likely to be oxidized than Fe, there is an increase in the amount of Si oxidized, which results in the oxidation of Fe being inhibited or results in a decrease in the amount of Fe oxidized. As a result, the layer of reduced Fe is insufficiently formed on the surface after reduction has been performed, or SiO2 exists on the surface of the steel sheet after reduction has been performed, which may result in a lack of hiding occurring in a chemical conversion coating.